The present invention relates to a golf ball having a core and a cover composed of at least one layer. More specifically, the invention relates to a golf ball which achieves satisfactory distances on shots with both drivers and middle irons used by professional golfers and skilled amateurs, and which also has an excellent durability to cracking when repeatedly struck.
Solid golf balls have a relatively simple inner structure composed of a core and a cover, although various golf ball properties such as rebound, feel, spin on approach shots and durability depend to a large degree on synergy between the core and the cover. In numerous disclosures to date, efforts have been made to optimize the rebound and feel of the ball, and also the spin rate of the ball on approach shots, by closely specifying the cross-sectional hardness of the core. Such art is described in, for example, the following technical literature: JP-A 2011-136020 (and the corresponding published U.S. Patent Application No. 2011/0159999), JP-A 2011-136021 (and the corresponding published U.S. Patent Application No. 2011/0159998), JP-A 2007-152090 (and the corresponding U.S. Pat. No. 7,273,425), JP-A 2008-194473 (and the corresponding U.S. Pat. No. 7,481,722), and JP-A 2010-214105 (and the corresponding U.S. Pat. No. 7,909,710).
In addition, numerous disclosures have been made on art which, in rubber compositions for golf ball cores, specifies the hardness profile of the core from the standpoint of the rubber formulation. For example, JP-A 2006-312044 (and the corresponding published U.S. Pat. No. 7,278,929) discloses art in which powdered sulfur is added so as to increase the hardness difference between the center and surface of the core to at least a given value. In addition, technical documents which describe rubber formulations created in order to adjust the hardness profile of the core include, for example, JP-A 2006-167452 (and the corresponding U.S. Pat. No. 7,276,560), JP-A 2006-289074 (and the corresponding U.S. Pat. No. 7,381,776), JP-A 2002-000765 (and the corresponding U.S. Pat. No. 6,679,791), JP-A 2010-188199 (and the corresponding U.S. Pat. No. 6,679,791, which is the same as the preceding), JP-A 2007-167257, JP-A 2008-68077 (and the corresponding U.S. Pat. No. 7,335,115), JP-A 2008-119461 (and the corresponding U.S. Pat. No. 7,300,362), and JP-A 2011-217857 (and the corresponding U.S. Patent Application No. 2011/0250991).
However, although a certain degree of improvement can be expected for conventional rubber formulations and core hardness profiles, even further increases in distance and improvements in durability are desired. Lately, research and development on golf balls has been especially intense and, in order to secure a competitive advantage with the ball, there has existed a desire to raise the level of the overall properties of the ball.